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From: guy@sun.uucp (Guy Harris)
Newsgroups: net.arch
Subject: Re: Re: Cleverness in the WE-32001
Message-ID: <2546@sun.uucp>
Date: Fri, 2-Aug-85 03:01:20 EDT
Article-I.D.: sun.2546
Posted: Fri Aug  2 03:01:20 1985
Date-Received: Sun, 4-Aug-85 06:12:26 EDT
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> This also may be a holdover from the past.  Most of the hardware
> implementations that I know about which use the 8086 and its
> descendants have separate I and D space.  In that case PC relative
> addressing for data objects won't work.

I looked at the "iAPX 286 Programmer's Reference Manual" and don't see any
PC-relative (or, in 80*8[68]-family terms, IP-relative) addressing modes.
That's why PC relative addressing for data objects won't work on the 8086
and its descendants - it doesn't exist.

The PDP-11s with separate I&D space use I space for references to
"instructions, index words, absolute addresses, and immediate operands" -
this implies that any operand references using the PC as an index register
other than immediate operands and absolute addresses use D space, so
PC-relative addressing for data objects will work.

The M68000 and M68010, however, have the PC as a register separate from the
general-purpose registers, and all references relative to it are "classified
as program references", which means they refer to I space if the hardware
has separate I&D space.

	Guy Harris